Masahiro Kodama

An application of cosmic-ray neutron measurements to the determination of the snow-water equivalent

Abstract Application of the water absorption character of cosmic-ray neutrons to the determination of the water equivalent depth of a snow cover is shown. We call this device the cosmic-ray snow gauge. The attenuation curves of the cosmic-ray snow gauge set under or in water or snow are experimentally obtained. Continuous field observations of the snow depths during four snow seasons demonstrate the effectiveness of the present device even for a snow cover of more than 1 m water equivalent, where the relative accuracy is found to be a few percent of the water depth. Possible effects of primary cosmic rays, barometric pressure and water content of soil upon the determination of snow depths are discussed.

Application of atmospheric neutrons to soil moisture measurement

This paper describes the possibility of continuous remote sensing of the moisture content of soil using atmospheric neutrons produced by cosmic radiations near the ground surface. Using polyethylene-moderated BF3 neutron counters at several different underground depths, we measured time variations of the neutron fluxes, to examine their responses to soil moisture changes quantitatively. From the close correlations between neutron fluxes and soil moisture contents, we show that the fluxes of the underground neutrons with energies from the cadmium threshold of 0.025 eV to about 106 eV, measured at a depth of 20 cm, are affected most sensitively by the moisture content of the soil near the same depth. Their fractional change is represented by a regression coefficient of 1% per unit percent of soil moisture change for a range from 33% (∼2.8 pF) to 52% (∼1.9 pF) at the 20-cm depth. This neutron technique promises to be a simple and reliable measurement that depends on the counting statistics of neutrons.

Feasibility studies of “polar patrol balloon”

Abstract Engineering and meteorological feasibilities of a circum-south-polar ballooning project, called “Polar Patrol Balloon (PPB)”, for space and geophysical researches are studied. We plan to use zero-pressure balloons mounting an auto-ballasting system, utilizing the non-sunset condition in mid-summer. PPB will be launced to a level higher than 30 km from an observational base in the Antarctica and come back there by a circumpolar wind. It is predicted that the circumpolar period may be a few weeks in the case of mid-summer 30 km-level flight and its meridional deviation after a circumpolar flight may be within a few hundreds kilometers. We have tested auto-ballasting and ARGOS-tracking, and are developing some on-board data accumulation systems and power supply. If we can collaborate with foreign bases, results of PPB should be much more successful especially concerning simultaneous observations at various latitudes.

Polar patrol balloon experiment in Antarctica

Abstract Since 1984 the National Institute of Polar Research, the Institute of Space and Astronautical Science and collaborative scientists have searched for feasibility of the long-term circumpolar balloon experiment called Polar Patrol Balloon (PPB) project. This project aims at establishing a station network in the stratosphere over the Antarctic region for geophysical and astrophysical observations. Three test flights in 1987 and 1990 at Syowa Station have convinced us that PPB would come back to the launching site with a good possibility. During 1991–1993 the PPB experiments will consequently be made for scientific researches at Syowa Station. This paper reports unique advantages of this PPB experiment and briefly reviews the past PPB experiments.

OBSERVATION OF COSMIC-RAY NEUTRON INTENSITY AT GEOMAGNETIC LATITUDE 25 N. PART I. ATMOSPHERIC EFFECTS

The intensity of cosmic-ray neutrons has been continuously observed at sea level and mountain. The value of absorption mean free path of neutron in the atmosphere, deduced from the barometer effect observed at each station, is 139±5 gr/cm 2 , while 136±2 gr/cm 2 was obtained from the intensity difference between two stations at different altitude.

MHD wave characteristics inferred from correlations between X-rays, VLF, and ULFs at Syowa Station, Antarctica and Tjornes, Iceland (L ∼ 6)

The Polar Patrol Balloon No. 6 (PPB#6) observed quasi-periodic pulsations of bremsstrahlung X-rays (E 30∼120 keV) in the daytime of 0855 UT (0914 MLT) ∼ 1630 UT (1614 MLT) on January 5, 1993, near Syowa Station, Antarctica (L∼6). The X-ray pulsations near the noon (1208:00 UT (1216 MLT) ∼ 1225:04 UT (1232 MLT)) include a period of about 260 sec, which corresponds to Pc 5 magnetic pulsations. It was found that at Syowa Station and Tjörnes, Iceland, which are both pair locations of the geomagnetic conjugacy, the X-ray pulsations are in correlation with the ULF-D pulsations. Also the Tjörnes VLF (2 kHz) pulsations correlated well with the X-ray pulsations of the period corresponding to Pc 5. It is probable that the VLF- and ULF-associated X-rays or precipitating energetic electrons in the vicinity of the L ∼ 6 shell are in synchronization with the electron-cyclotron resonance. Lastly, the so-called ballooning-mirror instability (the BMI) is a candidate to explain the compressional MHD waves that occur during the short time interval (1216 MLT ∼ 1232 MLT) in which the experimental results were interpreted.

27-DAY RECURRENCE OF COSMIC-RAY INTENSITY AT THE MINIMUM SOLAR ACTIVITY

There exists, in general, the close negative correlation between the 27-day recurrence tendency of cosmic rays and that of solar activity. Both recurrences have, however, the positive correlation during several months at the minimum solar activity. This phenomenon suggests that there may be at all times some solar emission of cosmic-ray particles, which is not observable except the short period at the minimum solar activity.

Ground Albedo Neutrons Produced by Cosmic Radiations

Day-to-day variations of cosmic-ray-produced neutron fluxes near the Earths ground surface are measured by using three sets of paraffin-moderated BF 3 counters, which are installed in the different locations; 3 m above ground, ground level and 20 cm under ground, respectively. Neutron flux decreases observed by these counters when snowcover exists show that there are upward-moving neutrons, that is, ground albedo neutrons near the ground surface. The amount of albedo neutrons is estimated to be about 40% of total neutron flux in the energy range 1–10 6 eV.

Observation of cosmic rays and auroral X-rays in the Polar Patrol Balloon experiment

Abstract The Polar Patrol Balloon (PPB#6) aimed at studying the elemental and isotopic composition of galactic cosmic rays, solar energetic particles cosmic gamma ray bursts and auroral X-rays, and was launched from Syowa Station Antarctica, by the 34th Japanese Antarctic Research Expedition on 5 Jan 1993. The balloon moved westward by 1.5 circumpolar rounds over Antarctica covering 6–13g/cm 2 atmospheric depth and 63°S–70°S geographic latitude. A total of 584 hours of observations was obtained. The balloon trajectory, the observing system and preliminary results of spectra and global distribution of cosmic radiation and auroral X-rays were described.